The Case of the Failing Bus Transmissions

Back in the 90s when I had just graduated with my mechanical engineering degree, I took a position with a small, privately-owned company building electric buses and shuttles. The company's shuttles were used in many locations such as Santa Barbara, Santa Monica, Burbank, and more. They made some good decisions in that they kept things fairly simple, and they managed a 12 to 14-hour run time on a single battery pack by continuously decreasing the weight of the bus while increasing the capacity of the battery.

While working there, I suddenly found myself responsible for seven half-finished bus projects all at once, and due to recent employee turnover, the projects were riddled with problems. There was no one I could turn to for any tribal knowledge, since all the former mechanical engineers were no longer around. I ran into one particularly nasty problem with the transmissions. They were failing with unusual regularity on our 26 food transit shuttles. There were dozens of these vehicles in service, so I needed to fix the problem quickly.

The drivetrain of the bus was fairly simple. There was an electric motor coupled to a chain transmission, such as the Morse/Emerson silent chain style. This was a single-gear reduction, flat-chain gearbox. The output of the transmission was coupled to a driveshaft, which in turn was coupled to a Dana 80 axle. The combination of the transmission and the differential on the axle provided the necessary gear reduction for the high-revving electric motor.

The first problem I came across was that technicians in the field were replacing the recommend gear oil in the transmissions with a thicker oil in order to "help" avoid failures. Thicker is better, right? The problem with this approach was that the oil they were using was not preferred for this style chain. The oil needed to have specific friction characteristics to lubricate the small pins that traverse the chain links. The heaver oil could not get into the crevices to properly lubricate the chain. Even if it could get in, it did not have the proper friction modifiers to lubricate the chain to the manufacturer's recommendations.

This improvised "fix" only served to accelerate the failures. It also increased the noise. Even though it is counterintuitive that thicker oil would make a transmission louder, the much-thinner recommended synthetic oil greatly reduced friction and allowed for a much quieter transfer of power.

Now that the oil dilemma was resolved, the transmissions were still failing. The original problem had yet to be discovered, and I still had some work on my hands to figure out why. As I observed a new shuttle being built, I noticed something interesting. The Dana axle was being installed pinion-down. Having spent time researching the axle with Dana, I knew it was a pinion-up design. The fabricators and assemblers in the shop were used to seeing axles in the pinion-down position and just assumed that was how it was to be installed. Adding to the problem, the production documentation was, unfortunately, not detailed enough to have helped avoid this simple mistake.

A really interesting case of good detective work. Chris' comments regarding the lack of "tribal knowledge" because so many of the engineers had moved on really struck me. That's a problem we will be seeing much more as baby boomer engineers continue to exit the work force. Obviously, there were flaws with this bus design, but the issue of capturing, storing, and disseminating domain expertise and so-called tribal engineering knowledge is going to be critical for companies going forward. And it's not just for future product development, but for continued maintenance and support.

Beth, you're right about the danger of losing tribal knowledge now that baby boomers are walking out the door with decades worth of knowledge that has become intellectual instinct. In some applications -- like plant operation -- that knowledge is getting captured in software.

This case has an extra twist -- Chris was able to figure out the oil problem as well as the upsidedown axle. That takes fresh thinking.

Not only fresh thinking, but good training and understanding of the basics. This is one thing that I noticed is lacking in some Engineering schools. There is a lot of theory, but the technical hands on is hit or miss with the students.

This solution was a great hands-on application of engineering knowledge. It also involves vendor application. Without knowledge of pin up or pin down installation, the transmissions might still be going backwards. It is sad to see that the company is no longer in business, but with such fundamental assembly problems and high turnover rates, it is not surprising.

Lack of documentation and employees who "know" how products are to be assembled can have tragic consequences. Gen. Chuck Yeager in his book "Yeager: An Autobiography" writes about an aircraft assembler who "knew" that aileron bolts were to be assembled "head up" despite what the drawing said; when the wing flexed under load the incorrectly-installed bolt caused the ailerons to lock up. After Yeager figured out the problem, at nearly the cost of his life, the company didn't have the heart to tell the assembler how many pilots he had killed.

The issue of "tribal knowledge" being lost was supposed to be addressed with ISO-9000 being implemented (documenting everything). Which is required if you have CE approval, which is required if you want to sell the product in Europe...

Obviously (with examples given), real life doesn't work that way.

Sorta like the "man" joke.... after having trouble assembling new toy, breaking down and digging the instructions out of the trash can. Just because it is documented doesn't mean someone will read it.

Our 13 years old grandson was visiting for the first time. we asked what he liked to do on his own, and he said he like to assemble projects. We bought him several motorized gadgets to play with. After working awhile on the first one, he came to me and said he could not get it to work properly. I saw that it was assempled wrong, but instead of pointing out the mistake, I asked if he had read the instructions. He said "no". I asked where the instructions were and he said "in the trash can". Biting my tongue, I suggested he retrieve and read them to find out what he did wrong. Further interaction revealed that he could not read well and was proud of it. He spent most of his time texting and listening to music on his iPod. Thus goes civilization.

Yes, samsuffy, that's a common problem. It was probably as big a problem 30 years ago as it is today.

I was schooled to keep instructions clipped to the receipt for all time. I was also schooled in the use of instructions by putting together model airplanes and ships as a kid. Then I moved on the Heath Kits. Then straight to the Allied catalog. I got a good refresher on instructions when I started to buy bikes and other assemble-it-yourself toys for my kids.

It seems obvious to me that the so called "tribal knowledge" partially caused the problem. You can't always depend on someone else for proper knowledge. Technical and theory type knowledge should be taught together in colleges and universities. Sometimes it is just how well an individual applies themselves. There is too much of doing just enough to get by these days. Just do just enough but get paid more. This could be a big reason America is falling behind in manufacturing. People should apply themselves more and then expect to get paid more. Todays main goal of most workers are to do as little work as possible because I don't get paid as much as my co-worker. There needs to be a more of a "sense of pride". Always do the best you can at whatever you do. Then the reward will come. Most of the time.

We need more explanation here: The axle was being installed upside down, with the pinion bwlow, like many passenger cars. How does that contribute to a transmission failure? by what mechanism does it cause a failure? I can easily immagine it causing an axle failure, but not transmission failure. Of course, incorrect assembly is often caused by incomplete drawing packages and build instructions.

So once again, what was ultimately found to be leading to all of the failures? And by what mechanism were the failures caused?

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